This invention relates to the auto-calibration of pressure transducers. In one preferred form, it relates to pressure transducers for use in apparatus for the provision of Continuous Positive Airway Pressure (CPAP) treatment to patients suffering from obstructive Sleep Apnea (OSA) and for use in apparatus for ventilating assistance.
CPAP is a well known treatment for the temporary relief of conditions including obstructive sleep apnea (OSA) and snoring. By this technique, air (or breathable gas) at a pressure elevated above atmospheric pressure is continuously supplied to the entrance of a patient""s airway (by the nasal and/or oral route) by means of known arrangements of masks or nasal prongs. The elevated air pressure acts as a pneumatic splint of the patient""s airway in the vicinity of the oro- and hypo-pharynx, reducing or eliminating the occurrences of apneas or hypopneas during sleep. A bi-level CPAP device, as opposed to a constant treatment level CPAP device, delivers two distinct pressures during the patient""s respiratory cyclexe2x80x94a relatively lower pressure during exhalation and a relatively higher pressure during inhalation. In another form, an automatically adjusting CPAP device may operate to provide a relatively low background pressure which increases to a therapeutic pressure on a needs basis, and preferably at a time to prevent the onset of an apnea.
The term xe2x80x9cCPAPxe2x80x9d used herein thus is to be understood as including constant, bi-level or adjusting forms of continuous positive airway pressure.
Common to all forms of CPAP apparatus is a nose, mouth or face mask which is fitted to a patient and connected to a flow generator via a flexible air delivery tube/conduit. The flow generator includes an electric motor driving a turbine to provide a supply of air or breathable gas for the administration of CPAP treatment during sleep. The range of positive air pressures supplied at the entrance to a patient""s airway typically is in the range 2-20 cm H2O. In the pressure regulation control of the flow generator it is usual to have a continuous measure of mask or flow generator delivery pressure, commonly achieved by locating a pressure sensing port at the mask or proximate the flow generator outlet.
In the clinical assessment of the severity of a patient""s OSA or upper airway syndrome condition, it is desired to identify the minimum possible CPAP treatment pressure that will alleviate the occurrence of partial or complete apneas during sleep. This is for the reason that the patient is required to expend respiratory effort in expiration against the positive airway pressure, hence it is preferable to minimise the work that must be done to ensure quality of sleep, and as follows administer only the minimal necessary CPAP treatment pressure. In this regard, it is important that the pressure transducer being used to measure the CPAP treatment pressure in control of the flow generator has satisfactory electro-mechanical characteristics so that the set-point CPAP treatment pressure does not vary significantly. It is known that a reduction of CPAP treatment pressure of as little as 1 cm H2O can nullify the therapeutic effect and result in a patient experiencing apneas during sleep.
There is, not unexpectantly, a direct correlation between the electro-mechanical performance of pressure transducers and price, hence the need for accurate pressure measurement is antagonistic towards the need to be able to manufacture CPAP apparatus at a cost that is acceptable to the marketplace. Commercially available pressure transducers, that are not extraordinarily expensive, operate in a small part of their pressure dynamic range in CPAP applications, meaning that there can be a 5-10% drift in the measured value with time due wholly to a pressure transducer operating in a xe2x80x98stretchedxe2x80x99 region of operation. Such a variation translates to a variation in CPAP treatment pressure of about 1-2 cm H2O. There further is market pressure for CPAP treatment to be determined to within an accuracy as low as 0.1 cm H2O.
It is therefore one preferred object of the invention to be able to avoid the need to incorporate expensive pressure transducers in CPAP apparatus and yet still maintain accurate monitoring of, and control over, CPAP treatment pressure.
A similar consideration applies for ventilators or apparatus for assisted ventilation that provide breathable gas to a patient at a controlled pressure. The gas is delivered to the patient, in the case of a ventilator, by way of a mask or an endotracheal tube. Patients with lung disease, neuromuscular disease, chest wall disease, or abnormalities of respiratory control may require ventilatory assistance. This is because they have various combinations of elevated airway resistance, stiff lungs and chest wall, ineffective respiratory muscles, or insufficient neural activation of the respiratory muscles. The need for ventilatory assistance is particularly common during sleep. Pressure controlled, time triggered ventilators, for example, deliver a relatively high inspiratory pressure (IPAP) for a fixed period of time (TI), and a relatively low expiratory pressure (EPAP) for another fixed period of time (TE). The cycle is then repeated indefinitely.
Pressure transducers typically are factory calibrated before delivery, to establish a zero pressure value (with respect to CPAP treatment pressure that is relative to atmospheric pressure) in terms of the transducer""s offset or bias. The xe2x80x9czero offset valuexe2x80x9d thus corresponds to atmospheric pressure. Even so, due to the inherent variations in the transduced pressure, and due to aging of the transducer and its temperature dependency, the preset offset value can vary by the equivalent of xc2x11 cm H2O leading to measurement error. This means that the patient must periodically return the CPAP apparatus to the manufacturer or servicer for re-calibration, else perform a re-calibration procedure themself, possibly requiring venting of the transducer to atmospheric pressure. It is therefore another preferred object of the invention to provide for auto-calibration of pressure transducer offset.
For convenience any reference to a xe2x80x9cmaskxe2x80x9d hereafter is to be understood as including nasal, oral or face masks, and nasal prongs.
The present invention is directed to methods and apparatus whereby one or more of the foregoing problems can be overcome or at least ameliorated.
Therefore, in a broad form the invention discloses a method for auto-calibration of the offset of a pressure transducer for use in CPAP or pressure regulated ventilation apparatus, the CPAP apparatus comprising a flow generator operable to supply breathable gas to a delivery tube in turn connected to a patient mask, and the pressure transducer measuring delivery pressure in the mask, delivery tube or flow generator, the method comprising the steps of:
determining whether the flow generator is operating;
determining whether there is no pressure activity sensed by the transducer; and
if both determinations are satisfied, accepting the output of the transducer as a calibrated pressure offset value representative of atmospheric pressure.
The invention further discloses a method for auto-calibration of the offset of a pressure transducer for use in CPAP or pressure regulated ventilation apparatus, the apparatus comprising a flow generator operable to supply breathable gas to a delivery tube in turn connected to a patient mask, and the pressure transducer measuring delivery pressure in the mask, delivery tube or flow generator, the method comprising the steps of:
determining whether no pressure activity is continuously sensed by the transducer over a predetermined period of time, and if so accepting the output of the transducer as a calibrated pressure offset value representative of atmospheric pressure.
The invention yet further discloses a flow generator for the supply of breathable gas comprising an electric motor driving a turbine, control circuitry, a pressure transducer to sense delivery pressure at or remote from said flow generator and whose electrical output is connected to said control circuitry, and sensing means connected to said control circuitry, and wherein said control circuitry is operable to determine from said sensing means whether the electric motor or the turbine are operating and from the pressure transducer whether there is no pressure activity, and if both determinations are satisfied, to accept the output of the transducer as a calibrated pressure offset value representative of atmospheric pressure.
The invention yet further discloses a flow generator for the supply of breathable gas comprising an electric motor driving a turbine, control circuitry, and a pressure transducer to sense delivery pressure at or remote from said flow generator and whose electrical output is connected to said control circuitry, and wherein said control circuitry is operable to determine from the pressure transducer whether there is no pressure activity continuously over a predetermined period of time, and if so, to accept the output of the transducer as a calibrated pressure offset value representative of atmospheric pressure.
An auto-calibration thus can be performed in the sense that there is no user/patient involvement nor manual activation, rather performance by the flow generator autonomously. If either determination is not met an auto-calibration is not performed.
The pressure offset value is applied to all subsequent pressure measurements to determine treatment pressure. Preferably, the two determinations are made over a plurality of successive instances and must both be satisfied on each instance before accepting the updated offset value.
In a preferred form, the current pressure value is compared against a preceding pressure value, and if differing less than a predetermined threshold then there is pressure inactivity. The preceding pressure value can be a running or moving average of such values.
The electric motor operation sensing means can be Hall-effect sensors integral of, or mounted to the electric motor. The flow generator can further comprise power supply means being controlled by the control circuitry and having control of the rotational speed of the electric motor.
The invention further discloses a flow generator as described, and a delivery tube coupled thereto and to a patient mask. The mask can be a nose, mouth or face mask. The pressure sensing port can be located at the turbine exit, a point along the tube or at the mask.
In one preferred form, the invention can be said to involve methods and apparatus for providing auto-calibration of pressure transducer offset that is implemented by continuously monitoring the flow generator electric motor to determine whether it is running and monitoring the pressure transducer for respiratory activity. If the motor is not running, and no pressure activity is detected, then the pressure measured by the transducer is determined to be atmospheric pressure, and so the electrical output from the transducer represents the zero pressure offset. The offset value at this point in time is stored to be subtracted from any subsequent pressure measurement values to determine treatment pressure.
In an alternate embodiment, auto-calibration of pressure transducer offset is implemented by determining whether there is no pressure activity sensed by the pressure transducer over a continuous period that is long compared with physiological events such as respiration and apnea. This single determination thus subsumes the separate determinations of motor operation and respiratory activity.
The zero offset can be updated whenever the opportunity arises, thus taking into account effects due to transducer ageing and temperature effects. In this way, the magnitude of a transducer""s offset error can be determined automatically without the need for additional hardware elements, such as a solenoid-operated valve venting the transducer to atmospheric. There also is no need for any user intervention in the periodic recalibration. This leads to a reduction in the cost of the hardware components of a CPAP or pressure regulated ventilation apparatus, and to a reduced manufacturing unit cost due to a reduction in labour required, for reason of there being no need to manually calibrate a transducer at the factory in advance of shipment, in combination resulting in reduced cost of the apparatus to the patient.
The improved pressure measurement accuracy gained also has therapeutic benefit, in that the CPAP or ventilation treatment will remain effective for a patient, in that the clinically-determined delivery pressure is maintained with accuracy.